The present invention relates to frequency modulated radio receivers and detectors, and more particularly, to a system and technique for recovering data from FM detectors.
Conventional FM detectors include structure to sense a frequency modulated carrier waveform and convert that frequency modulation into a voltage output representing the modulation. Such detectors are known to be temperature sensitive which produce temperature drift of the detector and temperature-related DC offsets during operation. While such characteristics are not particularly critical and may be tolerated in voice communication transmissions, they can produce unacceptable results when the FM detectors are used in connection with the transmission of FSK data.
In particular, when FSK data is transmitted and detected through a conventional FM detector, detector drift due to temperature variations may produce inaccuracies over time which affect the integrity of the data. This is especially true for those outputs from FM detectors which are capacitively coupled and include an extended series of signals representing digital 1's and 0's which can cause capacitor bleed. While this might be overcome by the use of a larger capacitance, the same would reduce response time to DC level changes representing data transmission. This is not acceptable in situations when an off-frequency, in-band, carrier creates a DC offset that must be eliminated instantaneously when valid data is received.
There is, therefore, a continuing need for providing circuits which may be used in connection with conventional FM detectors to improve their response when transmitting data and particularly to enable reliable recovery of FSK data. Accordingly, the present invention has been developed to overcome the limitations of the above-known and similar techniques, and to provide an improved data detector.